Hi all - any help is greatly appreciated.

My garage door opener (an older Sears/Craftsman 1990) has started
throwing off the chain. The sproket is in good shape (was replaced a
few years ago) but there seems to be a collar or bushing missing (or
broken) from the shaft that goes down into the housing. When garage
door begins to lift, the sprocket is pulled forward, tips slightly, and
throws the chain off. Do I need to replace the entire assembly? Do I
need a technician?

Thanks.
Bruce